1. Technical Field
The present invention relates to a vehicle occupant seat belt restraint system, and is particularly directed to a vehicle occupant seat belt restraint system having a seat belt pretensioner.
2. Background Art
Many different vehicle occupant seat belt restraint systems are known. Some systems include an actuatable pretensioner which, when actuated, draws the seat belt against a vehicle occupant. A typical vehicle occupant seat belt restraint system includes lap and shoulder belt portions which extend around an Occupant seated in a vehicle seat. When the vehicle experiences deceleration, such as occurs in a frontal collision, the occupant continues to move in the forwards direction of travel of the vehicle and presses against the lap and shoulder belt portions.
In a system which includes a pretensioner, a crash sensor is used to determine if the crash is above a predetermined threshold. If so, the pretensioner actuates, and the occupant presses against the seat belt earlier in the crash event than is the case without a pretensioner. As the occupant presses against the seat belt, energy is transferred to the seat belt. The energy is ultimately dissipated in the form of work done. Since the occupant presses against the seat belt earlier in the crash event, the dissipation of energy occurs over a longer time period, and the pretensioner therefore tends to lower the peak loads imparted to the occupant in a given crash. In high energy crash situations, even with the use of a pretensioner, the seat belt may cause excessive loads to be imparted to the occupant. Thus, after the pretensioner has deployed, it can be desirable to (i) allow the occupant to move as the occupant presses against the seat belt and (ii) dissipate energy transferred into the seat belt as a function of the occupant's movement by limiting the peak load.